Acorns And Mice Driving Unusual Lyme Disease Risks (ICEID 2)

The northeastern United States faces potentially “the worst year yet” for Lyme disease and other tickborne infections because of the periodic abundance of a little-noticed component of the disease’s complex ecology: acorns.

Dr. Richard S. Ostfeld of the Cary Institute of Ecosystem Studies in Millbrook, New York, explained during a presentation Tuesday at the International Conference on Emerging Infectious Diseases (ICEID) that a heavy crop of acorns in 2010 — a phenomenon known as a “mast year” — fueled a 2011 population bloom in white-footed mice, which stash acorns for winter food and begin breeding earlier in years when they are well-fed. That surge intersected with the two-year lifecycle of the ticks that transmit Lyme disease, for which mice are the key host, and this summer could produce a bumper crop of infected tick nymphs looking to bite large mammals — including humans.

“We’re already working with health departments” in Lyme-endemic areas to help craft messages to the public about the potential risk, Ostfeld said during his talk.

The prediction, which is based on earlier work by Ostfeld and colleagues (including these papers in 2006, 2005 and 2001) relies on the key role that white-footed mice (Peromyscus leucopus) play in perpetuating Lyme disease. That species, he said, appears to be the most competent reservoir for Borrelia burgdorferi, the bacterial cause of Lyme. Mice sustain the infection without cost to themselves, are frequently bitten by tick larvae, and groom off or otherwise kill the larvae at lesser rates than other small mammals that are bitten — allowing the larvae to drop off naturally and complete their transformation into tick nymphs that transmit infection in their second year of life.

Mice also can survive in much smaller areas than the larger animals, chiefly deer, that are usually blamed for perpetuating Lyme, Ostfeld pointed out. In sampling of “forest fragments” sliced up by development in three northeastern states, his team has not found a parcel in which mice did not thrive. Larger parcels with more balanced ecosystems, with natural mouse predators and larger mammals, actually tend to have lower Lyme density, he said.

Because of the yearlong gaps between bumper crops of acorns, mice, and then ticks, the reliable but irregular masting phenomenon could be used as an early-warning signal for Lyme exposure risks, Ostfeld pointed out. Oak trees mast roughly every three to five years, “and when you are in a mast year, you always know it,” he said.